Packaging of the ULSI chip is one of the most important steps in ULSI manufacturing, contributing significantly to the overall cost, performance and reliability of the packaged chip. As semiconductor devices reach higher levels of integration, packaging technologies such as chip bonding have become critical. Packaging of the chip accounts for a considerable portion of the cost of producing the device and failure of the package leads to costly yield reduction.
Some chip bonding technologies utilize a copper bump attached to a contact pad (chip bond pad) on the chip to make an electrical connection from the chip to the package. For example, new packaging methods including area array mounting include BGA (Ball Grid Array) and CSP (Chip Scale Package) methods where semiconductor chips are mounted on a substrate. In flip chip bonding, bumps are usually formed beforehand on the bonding pads of a semiconductor chip and the bumps are then interfaced with the terminals located on an interconnect substrate followed by, for example, thermo-compression bonding.
For example in the liquid crystal display panel art, driver chips must be mounted on a glass substrate. A mounting technology known as “chip on glass has emerged as a cost effective technique for mounting driver chips using a flat-top metal bump, for example a copper bump. Copper bumps may be formed by, for example, electroless or electrodeposition methods of copper over layers of under bump metallization (UBM) formed over the chip bonding pad. The copper bump (column) is typically formed within a mask formed of photoresist or other organic resinous material defining the bump forming area over the chip bonding pad.
In the electroless plating method, a metal is catalytically reduced onto a plating surface without the application of a power source. However, possible combinations of the metal base and the plating liquid are limited, and a plating rate is relatively low. Therefore, the electroless plating method is frequently not suitable for formation of metal films having a thickness in the micrometer range.
In some applications is desirable to form relatively thick copper bumps, for example, to improve the bonding strength in extreme environments where the bond is subjected to extraordinary stresses. For example, it is frequently desirable to form a copper bump having a thickness (height) of about 60 microns.
One problem with using copper metal to form bumps is the tendency of copper to oxidize at the surface to form a high electrical resistance copper oxide. Approaches thus far in the prior art have been directed at electroless and immersion plating of protective layers over the copper bump. Electroless an immersion plating approaches have been used since the process is relatively easy to implement requiring a solution bath and appropriate solution chemicals.
However, electroless and immersion plating is frequently unreliable it reliability frequently being device dependent in that the surface area and topography to be plated varies from device to device. As a result, mass transport of precursors to catalytically react at the plating surface is locally affected by the surface area to be plated as well as the feature topography which affects boundary layer concentrations and diffusion at the plating surface, frequently resulting in non-uniform thicknesses of plated or immersion plated metals. Mass transport properties in an electroless plating solution have proven to be difficult to adequately control. As the size and the topography of the plated surface changes, for example with varying bump sizes and densities, the difficulty of reproducing uniform and reliable metal films increases, frequently resulting in unacceptably low yields of about 10 to about 20 percent. For example it is difficult to monitor the thickness of metal layers being deposited making deposition control difficult.
These and other shortcomings demonstrate a need in the semiconductor processing art to develop an improved method for forming copper bumps including preventing subsequent copper bump oxidation.
It is therefore an object of the invention to provide an improved method for forming copper bumps including preventing subsequent copper bump oxidation in addition to overcoming other shortcomings and deficiencies in the prior art.